The invention herein described was made in the course of or under a contract or subcontract thereunder (or grant) with the Department of the Navy.
1. Field of the Invention
This invention relates generally to thermal spraying of powdered materials using high velocity oxy-fuel processes and specifically to a process or article applying a layer of metallic material over a superalloy shroud substrate in order to achieve an improved life expectancy.
2. Discussion of the Prior Art
High velocity oxy-fuel (xe2x80x9cHVOFxe2x80x9d) is a well-known commercial process used for application of a metal as a coating. Metal powder, typically an alloy powder, is applied by melting the powder at flame temperatures that are well below the temperatures required to melt ceramics. The melted powder is directed at a substrate and has high particle velocities. The HVOF process produces a densely deposited coating.
One of the applications for the HVOF process is for deposition of a metallic layer over the substrate of an article that is used in operating environments that are thermally and chemically hostile, such as the environment within a gas turbine engine. Such a layer is formed from high temperature, oxidation-resistant alloys including nickel-based superalloys, cobalt-based superalloys and MCrAlY alloys in which M can be iron, cobalt, nickel and combinations thereof.
When MCrAlY alloys are deposited as metallic layers for turbine section components of gas turbine engines, the HVOF process is preferred because it can provide a suitable coating at less expense. However, in order to apply a metallic layer to a substrate using the HVOF process that will properly adhere at elevated temperatures and under high stresses, it is necessary to properly prepare the substrate prior to metal application. Substrates that will utilize a metallic layer applied by the HVOF process generally are roughened to a surface finish of about 125 RMS or rougher. The roughened surface finish is required to improve the adhesion of the layer applied by the HVOF process. It is believed that the rough surface finish is initially required to provide a mechanical adhesion component to the attachment of the metallic layer to the substrate. The lower temperature of application of a layer using thermal spray processes such as HVOF creates a weak metallurgical bond, and the additional strength required for the bonding of the layer to the substrate may not be fully developed initially because of the lower application temperatures. This is a distinction from other processes such as plasma spray processes in which a stronger metallurgical bond is developed between the substrate and the applied coat. Metallic layers applied by the HVOF process are frequently used on shrouds used for turbine applications. The preparation of a substrate for application of the metallic layer by the HVOF process generally is accomplished by grit blasting. The shrouds to which the metallic layers have been applied are then heat treated to promote diffusion. The heat treatment further develops the metallurgical bond of the HVOF-applied metallic layer to the substrate. Thus, the final superior adhesion of the layer of the coating is a result of both mechanical and metallurgical bonding. However, it has been found that grit blasting can embed blasting media in the surface of the substrate. Furthermore, the embedded media can adversely affect the adhesion of the coating to the substrate. Too large a concentration of grit at the interface between the HVOF-applied layer and the substrate can impede the diffusion and act as stress risers that may contribute to delamination of the applied layer as the coating is cycled in service.
What is needed is a substrate article that has a roughened surface, the surface roughness being greater than about 80 RMS, but that does not include embedded grit or dirt from a roughening operation that may adversely affect the adhesion of an HVOF-applied coating to the substrate. Alternatively, if a substrate article does include embedded grit or dirt from a roughening operation, the amount of grit or dirt is controlled in a manner so as not to affect the adhesion of an HVOF-applied layer to the substrate. When an article is roughened by grit blasting, what is needed is a method to effectively control the amount of embedded grit or dirt to assure that it is maintained below a critical level in order to assure effective adhesion of the HVOF-applied layer to the substrate upon application.
According to the present invention, there is provided an article having a layer of metal applied by a thermal spray process, such as the HVOF process, over a substrate with a roughened interface at the surface of the substrate, the interface having a predetermined cleanliness level so that after a diffusion heat treatment, the applied layer has an extended life in severe gas turbine service due to improved adhesion of the layer to the substrate. When the article is used for high temperature applications and encounters significant levels of stress, the strength of the interface can be a factor in preventing coating delamination that affects the life of the article.
The article of the present invention includes a substrate and a layer of an MCrAlY that is applied by the HVOF process over the substrate. In accordance with the present invention, the substrate is roughened to achieve a surface finish of at least 80 RMS before application of the bond coat so that the interface therebetween is also at least 80 RMS. The surface preparation is controlled such that a cleanliness of less than 25% oxide contamination is present between the roughened substrate surface and the HVOF-applied metal. The article is thus comprised of a substrate with a layer of MCrAlY overlying the substrate, the interface between the substrate and the layer having a surface roughness of at least about 80 RMS and oxide contamination of less than about 25%.
The present invention can be achieved by roughening the substrate by any convenient method that achieves a surface roughness of at least 80 RMS before applying an MCrAlY coating using the HVOF process. This may include any suitable chemical, metallurgical or mechanical method that will produce an article that has such a surface roughness. Whatever method is used to roughen the surface of the substrate, it must be controlled by appropriate methods so that the amount of foreign residual material that remains on the surface that can affect the adhesion of the layer to the substrate remains below a predetermined level. This assures that the interface between the substrate and the applied layer is sufficiently clean so that premature failures due to delamination do not occur.
An advantage to the present invention is that metallic layers applied by the HVOF process to a suitably cleaned substrate will have a both a longer life and less susceptibility to delaminating than will layers applied by the HYOF process to substrates not suitably controlled in accordance with the present invention.
Another advantage of the present invention is that well known methods for roughening a substrate can be used to achieve the required mechanical bond between a layer applied by the HVOF process and the substrate provided that appropriate processing to clean the substrate of undesirable and excess contamination, particularly in the form of oxides is undertaken.
Yet another advantage of the present invention is that inspection techniques can determine whether sufficient cleanliness of the interface between an HVOF-applied layer and a substrate has been achieved, so that the likelihood of articles not having the required cleanliness being placed into service is significantly reduced or eliminated.
Another advantage of the present invention is that the use of a predetermined, quantifiable limit on cleanliness permits the use of statistical methods to control processing. Thus, a sampling plan for part inspection can be used to verify that the predetermined limits are not exceeded for parts placed into service. This ability to craft a sampling plan to determine the acceptability of a production lot of parts is important since interface cleanliness can only be determined by destructive examination, making 100% inspection an impossibility.
The present invention permits the use of cost effective HVOF metal application procedures and cost effective methods for cleaning the substrate prior to applying metal to the substrate in conjunction with statistically-based inspection procedures. These methods and procedures provide an article with assurance of an improved life expectancy in hostile environments in which operations occur at high temperatures, generally sustained temperatures above about 1800xc2x0 F. and, in certain circumstances, critical stresses above an endurance limit for the temperature of operation. Furthermore, the present invention permits the use of statistical analytical techniques to predict production lots of articles that are not sufficiently clean, and are likely not to achieve full life expectancy.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiments.